Its likely that you're tripping over an issue with the browser detection on IIS. Scott Hanselman wrote about this in the past with IE10, and the problem you're having does appear to mirror his description.
A hotfix available at the time, http://support.microsoft.com/kb/2600088, stated:
By default, ASP.NET uses sniffing technology for the user agent string to detect browsers. The browser definition files cover a certain range of browser versions. However, as the version numbers increase, ASP.NET might not recognize new versions of a browser by using the user agent string. In this case, ASP.NET might handle these versions as an unknown browser. For example, ASP.NET cannot recognize Windows Internet Explorer 10 that has the following user agent string:
Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; Trident/6.0)
However, this hotfix appears not to apply to IE11 due to a new format of user agent. There is a NuGet package named App_Browsers that may contain a fix, but until then you will have to write your own rule.
MSDN Browser Definition File Schema gives details on how to write a browser detection file; you will find your existing files in C:\Windows\Microsoft.NET\Framework\v4.0.30319\Config\Browsers.
According to MSDN Compatibilty Changes in IE11 Preview, the user agent for IE11 in Preview is:
Mozilla/5.0 (Windows NT 6.3; Trident/7.0; rv:11.0) like Gecko
which is not recognised by the standard IE regex (hence the problem you're seeing), however the following should work instead:
Trident\/7.0; rv:(?'version'(?'major'\d+)(\.(?'minor'\d+)?)(?'letters'\w*))(?'extra'[^)]*)
I've not tested it in a live environment, but this does parse the major and minor version correctly, which are key to solving the original problem - try adding this as another match in the file ie.browser
.
Note that a similar question was asked on MSDN recently - it may be worthwhile following and contributing to that.